Miocene deposits in western Amazonia and adjacent areas of South America harbour a diverse suite of endemic corbulid bivalves, commonly referred to as Pachydontinae, that show a wide variety of morphologies. Especially in the Miocene Pebas Formation (Peru, Colombia and Brazil), this group diversified spectacularly. Since these corbulids (a cosmopolitan marine and perimarine bivalve family) occur with freshwater taxa and yield isotope signals strongly indicative of freshwater settings, the success of this group in inland basins of Miocene northwestern South America is surprising. In this paper it is argued that a combination of adaptations to fluid bottom substrates, common dysoxia and high predation intensities explains their abundance, their morphological diversity and the paucity of freshwater bivalve groups, such as Sphaeriidae, Corbiculidae and Unionoidea.